The leader of ISIS, the fanatical Islamists who behead and crucify their enemies, has appeared in his first video, telling Muslims around the world they must obey him.

Abu Bakr al-Baghdadi had already proclaimed himself leader of a 'caliphate' that ISIS declared after seizing large swathes of Iraq and Syria.

The video appears to have been filmed on Friday during a sermon at a mosque in Mosul, Iraq's second city, which fell to ISIS last month as it tore through the country.

If it is genuine, it contradicts reports that Baghdadi had been killed or injured in an attack.

Baghdadi implores Muslims to 'obey' him during a sermon in a Mosul mosque

Wearing a black turban and robe and with a fan behind him to keep him cool, he says: "I am the wali (leader) who presides over you, though I am not the best of you, so if you see that I am right, assist me.

"If you see that I am wrong, advise me and put me on the right track, and obey me as long as I obey God in you."

According to translations, Baghdadi hailed the "victory" which he said had set the stage for the proclamation of the caliphate.

The video appears to show these worshipers attending the sermon

"God gave your mujahedeen brothers victory after long years of jihad and patience... so they declared the caliphate and placed the caliph in charge," he said.